1. Field of the Invention
The invention relates to a process for denitrogenation of a feedstock of a liquefied mixture of hydrocarbons, referred to by the abbreviation LNG, consisting chiefly of methane and also containing at least 2 mol % of nitrogen, in order to lower this nitrogen content to less than 1 mol %.
The gases which are supplied under the name of natural gases for the purpose of being used as fuel gases or as components of fuel gases are mixtures of hydrocarbons consisting chiefly of methane and generally containing nitrogen in a variable quantity which can reach 10 mol % or more.
It is commonplace to liquefy the natural gases on the site where they are obtained to produce liquefied natural gases (LNG), this liquefaction making it possible to reduce approximately six hundred times the volume occupied by a given molar quantity of gaseous hydrocarbon mixture, and to transport these liquefied gases towards the places where they are used by performing this transportation in large-sized thermally insulated storage vessels which are at a pressure equal to or slightly higher than atmospheric pressure. At the places where they are used, the liquefied gases are either vaporised for an immediate use as fuel gases or as components of fuel gases or else are stored in storage vessels of the same type as the transport storage vessels with a view to a subsequent use.
The presence of nitrogen in a significant quantity, for example greater than 1 mol %, in liquefied natural gas is detrimental because it increases the cost of transport of the given quantity of hydrocarbons and, moreover, it also reduces the calorific value of the fuel gas produced by vaporising a given volume of liquefied natural gas, and it is common practice to subject the liquefied natural gas before it is transported or before it is vaporised to a denitrogenation with a view to lowering its nitrogen content to an acceptable value, generally lower than 1 mol % and preferably lower than 0.5 mol %.
2. Related Art
The article by J-P. G. Jacks and J. C. McMillan entitled "Economic removal of nitrogen from LNG" and published in the journal Hydrocarbon Processing, December 1977, pages 133 to 136, describes, among other things, a process for denitrogenation of liquefied natural gas by stripping with reboiling in a denitrogenation column. In such a process (cf. FIG. 3) an LNG feedstock at a pressure above atmospheric pressure is subjected to cooling by indirect heat exchange and then decompression to a pressure close to atmospheric pressure, the cooled LNG feedstock is introduced into a denitrogenation column comprising a plurality of theoretical fractionation stages, an LNG fraction is withdrawn at the bottom of the denitrogenation column and the said fraction is employed to carry out the indirect heat exchange with the LNG feedstock to be treated, then, after the said heat exchange, this fraction is reinjected into the denitrogenation column as a reboiling fraction, this injection being carried out below the last bottom tray of the denitrogenation column, a gaseous fraction rich in methane and nitrogen is removed at the top of the denitrogenation column and a denitrogenated LNG stream is drawn off at the bottom of the said column. The gaseous fraction rich in methane and nitrogen collected at the top of the denitrogenation column is compressed after recovery of the negative calories which it contains to form a fuel gas stream which is employed on the site which includes the denitrogenation plant.
A major disadvantage of the denitrogenation process such as that cited above lies in the fact that the quantity of fuel gas obtained from the gaseous fraction rich in methane and nitrogen which is collected at the top of the denitrogenation column is much greater than the site requirements, generally a natural gas liquefaction site, on which the denitrogenation unit is present. If the denitrogenation is conducted so as to make the methane content of the fuel gas produced correspond to the requirements of the plant, the gaseous fraction removed at the top of the denitrogenation column, and consequently the fuel gas corresponding to it, contain a large quantity of nitrogen, which can be greater than 50 mol % in some cases. In order to burn such a fuel gas it is necessary to resort to a burner technology adapted to fuel gases of low calorific value, and this results in technological problems when it becomes necessary to replace the said fuel gas with a natural gas of high calorific value.
German Patent Application No. 3,822,175, published on 4.1.90, relates to a process for denitrogenation of natural gas, in-which the natural gas at elevated pressure is cooled, after separation of the high boiling point compounds which it contains, by indirect heat exchange, and then decompressed to a pressure of a few bars to produce a liquid natural gas phase which is introduced into a denitrogenation column operating at a pressure of a few bars, the said column producing, at the top, a nitrogen-rich gaseous fraction and, at the bottom, a denitrogenated LNG stream. In this process a first and a second liquid fraction are withdrawn from the denitrogenation column at levels of this column which are situated between its middle part and its lower part and below the level of introduction of the liquid natural gas phase, and these fractions are employed to carry out the indirect heat exchange resulting in the cooling of the natural gas, and then the said fractions are reinjected into the denitrogenation column after the said heat exchange. The reinjection of each fraction is performed at a level of the denitrogenation column which is situated below the level of withdrawal of this fraction and so that the level of reinjection of the topmost withdrawal fraction is situated between the levels of withdrawal of the two fractions.